1. Field of the invention
The present invention relates to a developer for developing an electrostatic image, used in a process by which an electrostatic image is developed to form an image, Such as electrophotography, electrostatic recording or electrostatic printing. It also relates an image forming method, a toner image fixing method and an image forming apparatus that make use of the developer.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are conventionally known for electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure or solvent vapor.
Various met hods or techniques have been developed in relation to the above final step, i.e., the step of fixing the toner image to a sheet such as paper. A method must commonly available at present is the pressure heating system making use of a heating roller.
The pressure heating system making use of a heating roller is e method of carrying out fixing by causing an image-receiving sheet to pass over a heating roller whole surface is formed of a material having a releasability to toner while a toner image surface of the former is brought into contact with the surface of the latter under application of a pressure. Since in this method the surface of the heating roller comes into contact with the toner image of the image-receiving sheet under application of a pressure, a very good thermal efficiency can be achieved when the toner image is melt-adhered onto the image-receiving sheet, so that fixing can be carried out rapidly. This method is therefore very effective in high-speed electrophotographic copying machines. In this method, however, since the surface of the heating roller comes into contact with the toner image under application of a pressure, part of the toner image may sometimes adhere and transfer to the surface of the fixing roller, which may re-transfer to the subsequent image-receiving sheet to cause an offset phenomenon, resulting in a contamination of the image-receiving sheet. Thus, it is essential in the heating roller fixing system that no toner is adhered to the surface of the heat fixing roller.
Accordingly, under existing circumstances, it is sought to provide a binder resin for toner, having a broad fixing temperature range and high anti-offset properties.
Research has been conducted on two-color copying machines or full-color copying machines, and many of which have been put into practical use. For example, Journal of Electrophotographic Society, Vol. 22, No. 1 (1983) and Journal of Electrophotographic Society, Vol. 25, No. 1, p. 52 (1986) issued reports on color reproduction and gradation reproduction.
However, images formed by full-color electrophotography presently put into practical use are not necessarily satisfactory to those who have become accustomed to seeing color images that are not directly compared with actual things or objects as in television pictures, photographs and color gravures or that are made artificially more beautiful than actual things.
In the full-color electrophotography, development is carried out plural times using plural kinds of toners with different colors, and toner layers are superposed on the same support to form a full-color image. For this reason, binder resins used for color toners are required to satisfy conditions as itemized below.
(1) Fixed toner must be brought into an almost perfectly molten state such that the form of toner particles can not be distinguished, in order that the fixed toner does not cause irregular reflection of light to hinder color reproduction. PA1 (2) Since the toner layers are superposed, the binder resin must be transparent so that a different color tone in a lower toner layer has is not affected. PA1 a toner containing a binder resin and a colorant, said binder resin being comprised of an AB-type block copolymer having segment-A and segment-B; PA1 said segment-A having a copolymer structure comprised of a styrene monomer and at least one monomer selected from the group consisting of an acrylic monomer, a methacrylic monomer and a diene monomer, and said segment-B having a copolymer structure comprised of a styrene monomer and at least one monomer selected from the group consisting of an acrylic monomer, a methacrylic monomer and a diene monomer, provided that said segment-A and segment-B have copolymer structures different from each other. PA1 bringing a charging member to which a voltage has been externally applied, into contact with a latent image bearing member to statically charge the latent image bearing member, said charging member having a conductive rubber layer and a release film formed at least at a portion of said conductive rubber layer coming into contact with said latent image bearing member; PA1 forming an electrostatic latent image on the latent image bearing member having been statically charged; and PA1 developing said latent image using a developer to form a toner image; said developer comprising a toner containing a binder resin and a colorant, said binder resin being comprised of an AB-type block copolymer having segment-A and segment-B; said segment-A having a copolymer structure comprised of a styrene monomer and at least one monomer selected from the group consisting of an acrylic monomer, a methacrylic monomer and a diene monomer, and said segment-B having a copolymer structure comprised of a styrene monomer and at least one monomer selected from the group consisting of an acrylic monomer, a methacrylic monomer and a diene monomer, provided that said segment-A and segment-B have copolymer structures different from each other; PA1 transferring the developed toner image to a transfer medium from said latent image bearing member; and PA1 cleaning the surface of said latent image bearing member to remove toner remaining untransferred. PA1 said developer for froming said toner image comprises a toner containing a binder resin and a colorant, said binder resin being comprised of an AB-type block copolymer having segment-A and segment-B; said segment-A having a copolymer structure comprised of a styrene monomer and at least one monomer selected from the group consisting of an acrylic monomer, a methacrylic monomer and a diene monomer, and said segment-B having a copolymer structure comprised of a styrene monomer and at least one monomer selected from the group consisting of an acrylic monomer, a methacrylic monomer and a diene monomer, provided that said segment-A and segment-B have copolymer structures different from each other; and PA1 at least one of said pair of rollers opposingly brought into pressure contact comprises a roller substrate, an elastic layer provided on said roller substrate, an oil-resistant layer provided on said elastic layer and an anti-offset layer provided on, said oil-resistant layer; these layers having an elongation that is larger by degrees in order of the anti-offset layer, the oil-resistant layer and the elastic layer. PA1 a latent image bearing member capable of bearing an electrostatic latent image; PA1 a charging;means for statically charging said latent image bearing member; PA1 a latent image forming means for forming an electrostatic latent image on the latent image bearing member having been statically charged; PA1 a developing means for developing said electrostatic latent image to form a toner image on said latent image bearing member; PA1 a transfer means for transferring said toner image to a transfer medium from said latent image bearing member; PA1 a cleaning means for cleaning the surface of said latent image bearing member to remove toner remaining thereon untransferred; and PA1 a fixing means for fixing the toner image transferred to said transfer medium by the action of heat and pressure; wherein; PA1 said developing means retains a developer comprising a toner containing a binder resin and a colorant, said binder resin being comprised of an AB-type block copolymer having segment-A and segment-B; said segment-A having a copolymer structure comprised of a styrene monomer and at least one monomer selected from the group consisting of an acrylic monomer, a methacrylic monomer and a diene monomer, and said segment-B having a copolymer structure comprised of a styrene monomer and at least one monomer selected from the group consisting of an acrylic monomer, a methacrylic monomer and a diene monomer, provided that said segment-A and segment-B have copolymer structures different from each other.
As stated above, when used in monochrome copying machines, binder resins used for toner are required to give a broad fixing temperature range and high anti-offset properties. When used in full-color copying machines, binder resins are required not only to have a broad fixing temperature range but also to be transparent and to give a flat image surface when images are fixed.
In recent years, in the field ranging from monochrome copying machines to full-color copying machines, variety is also required, e.g., to take copies at higher speed, to shorten the heat-up time of the heating roller and to decrease power consumption.
In order to satisfy these requirements, it is necessary to provide a binder resin for a toner that permits low-temperature fixing, and also, as stated above, which provides a broad fixing temperature range, has an excellent transparency, and can provide a flat image surface when images are fixed.
Meanwhile, one can consider a method making use of pressure fixing toners. In this method, the binder resin can not melt when the toners are used as toners for full colors in which three or four colors are superposed to effect color reproduction, so that color-mixing performance becomes poor to give a dull, chroma-poor image. Hence, in the fixing step, heat must be applied to the extent that the binder resin can melt and achieve color mixture.
Only for the purpose of achieving low-temperature fixing, it is possible to decrease melt viscosity of binder resins for toners. For example, there is a method in which the molecular weight of resin or the glass transition point thereof is lowered. This method, however, may result in a poor storage stability of toner which tends to cause phenomena such as blocking between toners and melt-adhesion of toner to a developing drum.
Hitherto, for the purpose of expanding the fixing temperature range of vinyl polymers, methods in which an anti-offset agent is used are disclosed in Japanese Patent Applications Laid-open No. 58-14148, No. 58-72948, No. 59-174855, No. 59-174856 and No. 60-123855, and Japanese Patent Publications No. 52-3304, No. 52-3305, No. 55-52574 and No. 58-8505. These, however, are supplementary means, and may damage the transparency of the toner when applied to monochrome toners or may bring about a poor color mixing performance when applied to full-color toners.
Japanese Patent Applications Laid-open No. 56-158340, No. 58-86558, No. 58-203453, No. 59-88748, No. 59-226358, No. 60-45259, No. 60-45261 and No. 60-46566 and Japanese Patent Publication No. 60-2411 disclose binder resins for a toner that have a low-molecular weight component and a high-molecular weight component. Use of such resins has made it possible to expand the fixing temperature range to a certain extent, but, on the other hand, causes the problem that grindability is lowered or melt viscosity becomes excessively high at the time of heat kneading, because of the presence of high-molecular weight components such as gels. Particularly when such binders are used in full-color toners, a problem arises in that the smoothness of image surfaces when images are fixed is damaged, resulting in a poor color mixing performance.
U.S. Pat. No. 4,925,765 discloses a negative solid block toner wherein an AB type, BAB type or ABA type block copolymer is used as a charge control agent.
In this block copolymer, a copolymer comprising acrylic monomers or a copolymer comprising methacrylic monomers is used as segment-A and a copolymer comprising monomers selected from the group consisting of styrene, a substituted styrene, butadiene, and an acrylate and/or a methacrylate as segment-B.
It can be presumed that use of such a block copolymer as a binder resin for a toner causes the problems that the grindability in the preparation of the toner becomes poor which makes particle size distribution broad and also the environmental stability of toner becomes poor, because of the segment-A which is an acrylic copolymer or methacrylic copolymer.
Thus, it is very difficult to satisfy at the same time fixing at a low temperature, to expand the fixing temperature range and to maintain the properties of toner, i.e., storage stability, fluidity, durability, transparency, and smoothness of fixed-image surface.